Radioactive water spilled from pools holding spent nuclear fuel rods at the Onagawa power plant in Miyagi Prefecture following the strong earthquake late Thursday, the nuclear safety agency said Friday.

At the crisis-hit Fukushima Daiichi nuclear power plant or at another plant in Fukushima Prefecture, meanwhile, no new problems have surfaced since the magnitude 7.4 aftershock of the deadly March 11 quake.

While the spent fuel pools at the Onagawa plant and the Higashidori nuclear power station in Aomori Prefecture, both operated by Tohoku Electric Power Co, lost their cooling functions for 20 to 80 minutes after the quake, the temperature hardly rose, the Nuclear and Industrial Safety Agency said.

A small amount of contaminated water spilled on the floor was observed inside the buildings at all three reactors at the Onagawa plant, which has suspended operations since the mega earthquake and tsunami last month, according to the agency.

In all, water spilled or leaked at eight sections of the plant as a result of the 11:32 p.m. quake, according to Tohoku Electric.

As much as 3.8 liters of water leaked at one of them, with the highest level of a radioactive isotope—5,410 bequerels per kilogram—found in the spilled water on the floor beside a spent fuel pool in the building housing the No. 1 reactor.

A d v e r t i s e m e n t

A spent nuclear fuel disposal facility in the village of Rokkasho, Aomori Prefecture, lost external power supply and switched to an emergency generator but power was restored at 9:44 a.m. Friday, according to the agency.

The Higashidori nuclear power plant in Aomori also got power from an emergency generator after the Thursday quake, but its external power supply was restored at 3:30 a.m. Friday, according to Tohoku Electric and the government’s countermeasure headquarters.

No changes in radiation readings have been observed at any of the facilities, including Fukushima Daini, a nuclear power plant just south of Daiichi, both operated by Tokyo Electric Power Co. Nor is there any information that radioactive materials have leaked outside due to the aftershock.

Higashidori’s only reactor was undergoing regular maintenance at the time of the temblor, and its fuel rods were not inside the core but were stored in a spent fuel pool, the agency said.

The Onagawa nuclear plant lost three of its four external power connections but one of them was restored on Friday morning, with its cooling system for the spent nuclear fuel pools temporarily stopped.

Tokyo Electric said no new abnormalities have developed in any of the six reactors at Fukushima Daiichi, which it has struggled to bring under control since the plant was crippled by the disaster and began spewing radioactive materials into the environment.

No workers at the plant were hurt in the aftershock, the power supplier said, adding that its damage control steps of pumping fresh water into the No. 1 to No. 3 reactors to prevent them from overheating and injecting nitrogen into the No. 1 unit to prevent hydrogen from exploding were unaffected.

The quake late Thursday, focused about 40 kilometers off the coast of Miyagi, jolted areas in Miyagi already hit hard by last month’s quake.

Meanwhile, Tokyo Electric continued to release relatively low-level radioactive water into the Pacific Ocean to make room for far more contaminated water that has flooded the basement of the No. 2 reactor’s turbine building.

Once the utility finishes releasing the low contaminated water, it will check a facility that had held the water to see if there are any cracks to ensure that there would be no leakage when the more contaminated water is diverted there.

The turbine building needs to be cleared of radioactive water to restore sustainable cooling systems for the No. 2 reactor, which lost normal cooling functions and suffered a partial meltdown of the core since the deadly quake last month, along with the No. 1 and No. 3 reactors.

The No. 4 reactor had no fuel in its core but suffered a loss of cooling functions for its fuel pool. The No. 5 and No. 6 reactors have since been brought to a stable condition.

On Friday, the power company reinforced the earth around a cracked pit, from which highly radioactive water had leaked into the sea before it was successfully plugged by injecting sodium silicate, a chemical agent known as ‘‘water glass.’‘

Since the leakage had stopped, the company observed about a 7-centimeter rise in the level of contaminated water in a vertical tunnel connected to the No. 2 reactor building, from which the tainted water is believed to have originated.